1. Field of the Invention
The present invention relates to a packet switching device and a fixed length packet transfer control method, and more particularly to an ATM (Asynchronous Transfer Mode) packet switching device having a congestion control function and a cell transfer control method in an ATM network.
2. Description of the Related Art
The transfer of fixed length packets (hereinafter referred to as a "cell") is described, for example, in "Data Communication Using ATM: Architectures, Protocols, and Resource Management", IEEE Communication Magazine, August 1994, p 24-33 and "SVC Signaling: Calling All Nodes", DATA COMMUNICATIONS, June 1995, p 123-128, and so on.
In an ATM network, a call (connection) is set up by signaling processing performed upon initiating the call along a communication path from a transmitting device (source terminal) to a receiving device (destination terminal) through a switching device (switch) serving as a transfer path for user cells, and the transfer of the user cells is controlled based on connection identification information attached to a header portion of each user cell.
A call setup procedure is described, for example, in ITU-T Standard Q.2931, and the call setup procedure is executed to set connection information to a transmitting device, each node (switch) on a path, and a receiving device. The connection information includes identifiers for identifying a call on each of links between the transmitting device and a switch, between switches, and between a switch and the receiving device, a traffic class indicative of a priority of cell transfer in a switch, and so on. The identifiers for identifying a connection (call) are referred to as "VPI" (Virtual Pass Identifier) and "VCI" (Virtual Connection Identifier) and set in a header portion of each cell as address information.
Each switch searches for connection information necessary for the switching operation based on VPI and VCI in each of input cells received through a transmission path. The connection information includes, for example, internal routing information (output port number), identifiers to be attached to an output cell (output VPI/VCI), a traffic class indicative of a cell priority in the switch, and so on.
The traffic class indicative of a cell priority is described, for example, in "Multimedia Traffic Management Principles for Guaranteed ATM Network Performance", IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, Vol. 8, No. 3, April 1990, p 437-446, and "Traffic Management for B-ISDN Services" IEEE Network, September 1992, p 10-19.
The traffic class indicative of a cell priority includes two: CBR (Constant Bit Rate) and VBR (Variable Bit Rate). CBR is a traffic class in which a predetermined cell transfer rate is contracted between a network and a terminal upon setting up a call so that the network side guarantees the cell transfer at the contracted transfer rate. VBR is a traffic class which tolerates a certain degree of statistical variations for a transfer rate contracted with a terminal. The scheme which controls traffic with a contract previously made between a network and a terminal is referred to as "preventive control".
As traffic for transmitting cells utilizing a remaining portion of a bandwidth allocated to another terminal in the above-mentioned CBR and VBR without making a special contract related to the transfer rate between a network and a terminal upon setting up a call, there is a group of traffic classes referred to as "best effort control". The transfer rate contract is not made primarily because it is difficult for a terminal outputting burst traffic to predict the characteristic of the traffic upon setting up a call.
The group of best effort control traffic includes a UBR (Unspecified Bit Rate) traffic class in which a network does not guarantee anything with respect to cell transfer, and an ABR (Available Bit Rate) traffic class which performs a feedback control between a network and a terminal during congestion to guarantee the prevention of cell loss. The ABR traffic class is described, for example, in "Rate-Based Flow Control Framework for the Available Bit Rate ATM Service", IEEE Network, March/April 1995, p 25-39.
A configuration of a switch for controlling a transfer in accordance with a traffic class is described, for example, in JP-A-6-197128. In a packet switching scheme described in this publication, each output port is provided with two output buffers, one for CBR traffic class and the other for VBR traffic class, and information indicative of an empty/full state of the two buffers is stored as table information corresponding to each output port, such that an input buffer control unit references the table information to determine a buffer for accumulating cells destined to each output port.
In this case, the output priority of cells accumulated in CBR buffer is ranked higher than that of the VBR buffer so that a communication delay time in a switch can be limited within a predetermined value for a group of cells of the CVR traffic which has strict restrictions to communication delay. In addition, if the CBR buffer does not have any free space, for example, cells may be accumulated in the VBR buffer, provided that it has some free space, to make good use of a bandwidth in the switch. Further, for supporting the ABR and UBR traffic classes, an output buffer supporting further traffic classes may be additionally provided in addition to the CBR and VBR traffic classes.
As described above, while several traffic classes have already been proposed in asynchronous communication, it is desired to perform a transfer control by further fractionizing the characteristics in each traffic class, in addition to controlling how a plurality of these traffic classes are properly used.
However, taking as an example the UBR traffic classes which do not give special guarantee to a cell transfer, when a network becomes congested, a conventional system does not have any means for controlling the Quality of Service belonging to these traffic classes.